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K.E. Hamilton, D.C. Pye; Variations in Corneal Young's Modulus Can Significantly Affect Goldmann Tonometry Estimates of Intraocular Pressure in Young Adults . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4451.
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There is strong evidence that the accuracy of Goldmann applanation tonometry estimates of intraocular pressure (IOP) are affected by corneal thickness, but other corneal properties such as Young’s modulus may also be important. The aim of this study was to determine the statistically normal range of corneal Young’s modulus in healthy eyes in vivo, and to establish if this variation is likely to have a clinically significant influence on IOP measurement.
Central corneal curvature (Reichert EyeChek autokeratometer), central corneal thickness (BVI ultrasonic pachymeter) and applanation IOP (Goldmann tonometer) were measured in one eye of 100 normal healthy human subjects (22.0±2.9 y) in vivo. The Orssengo–Pye algorithm was used to calculate the Young’s modulus of the corneas of these experimental subjects, and to provide an theoretical estimate of potential errors in Goldmann applanation tonometry estimates of IOP due to variations of Young’s modulus and corneal thickness.
The mean central corneal curvature was 7.73 mm (95% CI 7.22–8.24 mm), central corneal thickness was 539.2 µm (95% CI 478.7–599.8 µm) and applanation IOP was 14.26 mmHg (95% CI 8.58–19.94 mmHg). Corneal Young’s modulus was calculated to be 0.29 MPa (95% CI 0.17–0.40 MPa). According to the Orssengo–Pye model, the relationship between Young’s modulus and the error in applanation IOP is linear; in this study, the slope was 2.30 mmHg per 0.1 MPa (linear regression). An increase from the minimum to the maximum value of the calculated limits of agreement (95% CI) of Young’s modulus would cause an error in applanation IOP of 5.35 mmHg. The anticipated error at the extremes of the limits of agreement (95% CI) of corneal thickness was similar at 4.67 mmHg.
In young normal healthy subjects, physiological variations in corneal Young’s modulus are capable of causing clinically significant errors in Goldmann applanation tonometry estimates of IOP of a similar magnitude to those induced by corneal thickness.
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